This invention relates to a zone plate for focusing microwave energy and in particular to a phase correcting reflection zone plate for focusing microwaves. This invention also relates to apparatus and a method for manufacturing such a zone plate.
The use of zone plates for focusing microwaves is well known. One particular type of zone plate disclosed in "Millimeter-Wave Characteristics of Phase-Correcting Fresnal Zone Plates" by D. N. Black and J. Wiltse, IEEE Transactions on Microwave Theory and Technique Volume 35 No. 12 (1987) Page 1122-1128, is the phase-correcting Fresnel zone plate. Such a zone plate is shown schematically in FIG. 1 for quarter wave correction, although a phase correcting zone plate can be made for any wavelength fraction. The radius of each zone r.sub.n can be given by ##EQU1## where n is the zone number, f is the focal length of the zone plate, .lambda. is the wavelength of the radiation and P is an integer greater than 2. For quarter wave correction P=4. For such a zone plate both in and out of phase zones contribute to the energy at the focus thus increasing the efficiency compared to a conventional zone plate. The correction of the phase of the zones is achieved by changing the path length of the energy reflected from that zone. Thus the energy reflected from the zone 2a of the quarter wave zone plate of FIG. 1 would be out of phase with respect to the energy from the zone 3 by .lambda./4 at the focus, unless the pathlength was decreased or increased by .lambda./4. An increase in pathlength of .lambda./4 is achieved by providing steps .lambda./8 in depth. Thus zone 2a is .lambda./8 higher than zone 3 and zone 2b and 2c are .lambda./4 and 3.lambda./8 higher than zone 3 respectively. More generally, the different phases of the zones of the zone plate are stepped by d where EQU d=.lambda.o/2P
where .lambda.o is the free space wavelength of the radiation.
The construction of such a zone plate may be achieved by a number of manufacturing processes such as machining out of solid metal, stamping out of a thin metal sheet, moulding and subsequently metallising a plastic material or by vacuum forming plastics.